Dual Frequency global positioning system

Abstract

A global position system receiver recovers the L1 and L2 carriers, the C/A-code measurements and the L1 and L2 P-code measurements by (a) producing an estimate of the L1 carrier phase angle and synchronizing a locally generated C/A-code with the L1 signal using an L1 delay lock loop and controlling, based on the locally generated C/A code, an L1 P-code generator that produces a synchronized version of the P-code; (b) initializing an L2 P-code generator based on the phase of the P-code generated by the L1 P-code generator; (c) determining, for the L2 signal, an estimate of signal power that is adjusted to compensate for noise; (d) determining an L2 carrier phase angle; (e) tracking the L2 P-code by adjusting an L2 P-code generator until the estimate of signal power is maximized. The GPS receiver also resolves a 1/2 cycle ambiguity in tracking the L2 P-code by comparing demodulated L1 P-code bits and L2 P-code bits. If the bits do not match in the majority of instances, the receiver determines that it is tracking the L2 P-code with 1/2 cycle error and adjusts its estimate of the P-code phase accordingly. The receiver may determine the reliability of the independently tracked L2 signal, by determining an associated carrier-to-noise ratio.

What is claimed is: 
     
       1. A global position sensing receiver for receiving over L1 and L2 bands, respectively, L1 signals that are modulated both by a C/A-code and a P-code that is, at times, encrypted and L2 signals that are modulated by the P-code that is, at times, encrypted, the receiver including: A. means for producing a locally-generated version of the C/A-code that is aligned with the C/A-code in the received L1 signal;   B. an L1 P-code generator for producing a locally-generated version of the P-code that is phase-locked to the locally-generated version of the C/A-code;   C. an L2 carrier angle estimation means for producing an estimate of the phase angle of an L2 carrier;   D. an L2 P-code generator for producing a locally-generated version of the P-code;   E. a controller for controlling the operations of the L2 P-code generator, the controller receiving signals associated with a correlation between the locally-generated P-code and the received L2 signals, and adjusting the L2 P-code generator to align the locally-generated P-code with the L2 signals; and   F. a detector for resolving a 1/2-cycle ambiguity in tracking the L2 P-code, the detector including: i. a majority vote circuit for determining if a majority of bits of the received L2 P-code match or do not match corresponding bits of the received L1 P-code, and   ii. means for adjusting the carrier angle estimation means if the majority vote circuit determines that a majority of the L1 and L2 bits do not match.     
     
     
       2. The global position sensing receiver of claim 1, wherein the controller receives signals associated with the estimated phase angle of the L2 carrier. 
     
     
       3. The global position sensing receiver of claim 1 wherein the controller determines a carrier phase error estimate as: ##EQU3## where K 1  and K 2  are constants, I and Q are in phase and quadrature correlation signals, "sign I" represents the sign of the I signal, and S T  is an estimate of total signal power, and based on the error estimate adjusts the L2 carrier angle estimation means. 
     
     
       4. The global position sensing receiver of claim 3 wherein the controller determines total signal power as a sum of a. signal power in an inphase path minus inphase noise components, and   b. signal power in a quadrature path minus quadrature noise components.   
     
     
       5. The global position sensing receiver of claim 4 wherein the noise components are determined by a multiplying inphase and quadrature components of the L2 signal by an uncorrelated version of the P-code produced by the L2 P-code generator. 
     
     
       6. The global position sensing receiver of claim 4 wherein the controller, after lock is acquired, substitutes for the estimate of the total signal power the signal power in the inphase path minus noise. 
     
     
       7. The global position sensing receiver of claim 1 wherein the controller, to determine carrier phase tracking error, tracks arctan(Q*signI), where I and Q are inphase and quadrature correlation signals, and based on a determination of the phase error adjusts the L2 carrier angle estimation means. 
     
     
       8. The global position sensing receiver of claim 1 further including means for determining the reliability of the L2 signal by determining an associated carrier-to-noise ratio. 
     
     
       9. A global position sensing receiver for receiving over L1 and L2 bands, respectively, L1 signals that are modulated both by a C/A-code and a P-code that is, at times, encrypted, and L2 signals that are modulated by the P-code that is, at times, encrypted, the receiver including: A. means for producing a locally-generated version of the C/A-code that is aligned with the C/A-code in the received L1 signal;   B. an L1 P-code generator for producing a locally-generated version of the P-code that is phase-locked to the locally-generated version of the C/A-code;   C. an L2 P-code generator for producing a locally-generated version of the P-code;   D. correlation means for determining how closely the P-code produced by the L2 P-code generator and the P-code in the L2 signal are aligned, said means producing L2 correlation signals;   E. L2 carrier angle estimation means for providing an estimate of a carrier phase angle based on the correlation signals produced by the correlation means; and   F. a controller for controlling the operations of the L2 P-code generator and the L2 carrier angle estimation means, the controller receiving signals from the correlation means and the L2 Carrier angle estimate means, and adjusting the L2 P-code generator, to align the locally-generated P-code with the L2 signals, and adjusting the L2 carrier angle estimation means to phase lock said means to the L2 signal, the controller determining a carrier phase error estimate as: ##EQU4## where K 1  and K 2  are constants, I and Q are inphase and quadrature correlation signals, "sign I" represents the sign of the I signal, and S T  is an estimate of total signal power, the controller using the error estimate to adjust the L2 carrier angle estimation means.   
     
     
       10. The global position sensing receiver of claim 9 wherein the controller further includes means for determining the reliability of the L2 signals. 
     
     
       11. The global position sensing receiver of claim 9 wherein the controller determines total signal power as a sum of a. signal power in an inphase path minus inphase noise components, and   b. signal power in a quadrature path minus quadrature noise components.   
     
     
       12. The global position sensing receiver of claim 11 wherein the noise components are determined by multiplying inphase and quadrature components of the L2 signal by an uncorrelated version of the P-code produced by the L2 P-code generator. 
     
     
       13. The global position sensing receiver of claim 11 wherein the controller, after lock is acquired, substitutes for the estimate of the total signal power the signal power in the inphase path minus inphase noise components. 
     
     
       14. The global position sensing receiver of claim 9 wherein the controller, to determine carrier phase tracking error, tracks the product arctan(Q*signI), where I and Q are inphase and quadrature correlation signals. 
     
     
       15. The global position sensing receiver of claim 14 further including a detector for resolving a 1/2-cycle ambiguity in tracking the L2 P-code, the detector including: i. a majority vote circuit for determining if a majority of bits of the received L2 P-code match or do not match corresponding bits of the received L1 P-code, and   ii. means for adjusting the carrier angle estimation means if the majority vote circuit determines that a majority of the L1 and L2 bits do not match.   
     
     
       16. A global position sensing receiver for receiving over L1 and L2 bands, respectively, L1 signals that are modulated both by a C/A-code and a P-code that is, at times, encrypted, and L2 signals that are modulated by the P-code that is, at times, encrypted, the receiver including: A. means for producing a locally-generated version of the C/A-code that is aligned with the C/A-code in the received L1 signal;   B. an L1 P-code generator for producing a locally-generated version of the P-code that is phase-locked to the locally-generated version of the C/A-code;   C. an L2 P-code generator for producing a locally-generated version of the P-code;   D. correlation means for determining how closely the P-code produced by the L2 P-code generator and the P-code in the L2 signal are aligned, said means producing L2 correlation signals;   E. L2 carrier angle estimation means for providing an estimate of a carrier phase angle based on the correlation signals produced by the correlation means; and   F. a controller for controlling the operations of the L2 P-code generator and the L2 carrier angle estimation means, the controller receiving signals from the correlation means and the L2 carrier angle estimate means, and adjusting the L2 P-code generator, to align the locally-generated P-code with the L2 signals, and adjusting the L2 carrier angle estimation means to phase lock said means to the L2 signal, the controller determining a carrier phase error estimate based on an estimate of total signal power, S T , which is determined as a sum of   a. signal power in an inphase path minus inphase noise components, and   b. signal power in a quadrature path minus quadrature noise components, the controller using the error estimate to adjust the L2 carrier angle estimation means.     
     
     
       17. The global position sensing receiver of claim 16 wherein the noise components are determined by multiplying inphase and quadrature components of the L2 signal by an uncorrelated version of the P-code produced by the L2 P-code generator. 
     
     
       18. The global position sensing receiver of claim 17 wherein the controller determines a carrier phase error estimate as: ##EQU5## where K 1  and K 2  are constants, I and Q are inphase and quadrature correlation signals, "sign I" represents the sign of the I signal. 
     
     
       19. The global position sensing receiver of claim 16 wherein the controller, after lock is acquired, substitutes for the estimate of the total signal power the signal power in the inphase path minus inphase noise components. 
     
     
       20. The global position sensing receiver of claim 16 further including a detector for resolving a 1/2-cycle ambiguity in tracking the L2 P-code, the detector including: i. a majority vote circuit for determining if a majority of the bits in the L2 P-code match or do not match corresponding bits of the L1 P-code, and   ii. means for adjusting the carrier angle estimation means if the majority vote circuit determines that a majority of the L1 and L2 bits do not match.